Fluctuations in the Cosmic Microwave Background

The cosmic microwave background is the afterglow radiation left over from the hot Big
Bang. Its temperature is extremely uniform all over the sky. However, tiny temperature
variations or fluctuations (at the part per million level) can offer great insight into
the origin, evolution, and content of the universe.

If you were approaching the Earth on a spaceship, the first thing you would notice is
that the planet is spherical. As you drew closer to the Earth, you would see the surface
divide into continents and oceans. You would need to study the Earth's surface very
carefully to see the mountains, cities, forests and deserts that cover the continents.

Similarly, when cosmologists first looked at the microwave sky, thirty years ago, they
noticed it was nearly uniform. As observations improved, they detected the
dipole anisotropy. Finally, in 1992, the Cosmic
Background Explorer (COBE)
satellite made the first detection analogous to seeing "mountains on the surface of
the Earth": it detected cosmological fluctuations in the microwave background
temperature. Several members of the WMAP science team help lead the COBE program and build
the spacecraft. COBE's detection was confirmed by the
Far InfraRed Survey (FIRS)
balloon-borne experiment.

Comparison of COBE and WMAP sky images

Fluctuations seen by COBE

Fluctuations seen by WMAP (Simulated)

In the comparison of the images above, images on the left produced by the COBE science team, show three false color images of the
sky as seen at microwave frequencies. The images on the right show one of our computer simulations of what the WMAP experiment detects. Note that WMAP detects much finer features than are visible in the COBE maps of the sky.
This additional angular resolution allows
scientists to infer a great deal of additional information, beyond that supplied by COBE,
about conditions in the early universe.

The orientation of the maps are such that the plane
of the Milky Way runs horizontally across the center of each image. The top pair of figures show
the temperature of the microwave sky in a scale in which blue is 0 Kelvin (absolute zero)
and red is 4 Kelvin. Note that the temperature appears completely uniform on this scale.
The actual temperature of the cosmic microwave background is 2.725 Kelvin. The middle
image pair show the same map displayed in a scale such that blue corresponds to 2.721 Kelvin and
red is 2.729 Kelvin. The "yin-yang" pattern is the dipole anisotropy that
results from the motion of the Sun relative to the rest frame of the cosmic microwave
background. The bottom figure pair shows the microwave sky after the dipole anisotropy has been
subtracted from the map. This removal eliminates most of the fluctuations in the map: the
ones that remain are thirty times smaller. On this map, the hot regions, shown in red, are
0.0002 Kelvin hotter than the cold regions, shown in blue.

There are two main sources for the fluctuations seen in the last figure:

Emission from the Milky Way dominates the equator of the map but is quite small away from the equator.

Fluctuating emission from the edge of the visible universe dominates the regions away from the equator.

There is also residual noise in the maps from the instruments themselves, but this noise is quite small compared to the signals in these maps.

These cosmic microwave temperature fluctuations are believed to trace fluctuations in
the density of matter in the early universe, as they were imprinted shortly after the Big
Bang. This being the case, they reveal a great deal about the early universe and
the origin of galaxies and large scale structure in the
universe.